The universe expanded from a hot dense state, and is still expanding.
Standard Big Bang Model
The scientific consensus model of cosmic history from the Planck time onward.
An extraordinarily hot, dense state. All matter, all energy, all of what will become space, compressed into effectively nothing.
Looping ambient scene for Standard Big Bang. The foundation of modern cosmology. The universe began from a hot dense state, has been expanding and cooling, and we observe its history through the CMB, galaxy redshifts, and primordial element abundances.
§1 · The claim, in one sentence
Our universe began approximately 13.8 billion years ago as an extraordinarily hot, dense state, and has been expanding and cooling ever since.
§2 · Why it might be true
The Big Bang model is the foundation of modern cosmology. Lemaître proposed in 1927 that the universe began from a "primeval atom." Over the following decades, Gamow, Alpher, and Herman worked out the physics of nucleosynthesis: the formation of the lightest elements in the first few minutes. The 1965 discovery of the cosmic microwave background by Penzias and Wilson, a faint 2.7 K glow filling all of space, provided decisive evidence.
The Big Bang is not an "explosion in space", it is the expansion of space itself. Every point in our observable universe was, in the past, far closer to every other point. As space expanded, the universe cooled. Matter condensed. Stars formed. Galaxies clustered.
A crucial point: the Big Bang model describes physics from approximately 10⁻⁴³ seconds onward. It does not claim to explain what happened before, or what "caused" it. The singularity at t = 0 is where the mathematics breaks down, it is the boundary of what the theory can describe.
The family stance
Our universe began approximately 13.8 billion years ago and has been expanding ever since. The model is agnostic about what existed before, that question is for the Chapter 1 families.
§2.5 · Evidence
- CMB discovered 1965, confirmed by COBE, WMAP, Planck
- Hubble redshift confirmed in countless surveys
- BBN abundance predictions match observations within 1%
- Deep-field surveys show cosmic evolution
§3 · What you'd need to test it
- Expansion of space (Hubble redshift of galaxies)
- Cosmic microwave background at ~2.7 K
- Primordial nucleosynthesis abundance ratios
- Large-scale structure forming from initial perturbations
§4 · Where it breaks
- Does not address what preceded or caused it
- Horizon and flatness problems require inflation patch
- Singularity at t=0 is a mathematical breakdown
Go deeper
The expansion of the universe is described by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, with the scale factor a(t) evolving according to the Friedmann equations.
Early epochs: Planck era (t < 10⁻⁴³ s), grand unification (t ~ 10⁻³⁶ s), electroweak symmetry breaking (t ~ 10⁻¹² s), QCD phase transition (t ~ 10⁻⁶ s), primordial nucleosynthesis (t ~ 1-1000 s), recombination and CMB release (t ~ 380,000 yr).
Standard Big Bang is incomplete: it does not explain why the universe is so flat, homogeneous, and free of monopoles. Inflation is the widely accepted patch.
▸§5 · Who built it, and when(5 sources, 5 established)
- EstablishedLemaître (1927). 'Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques.' Ann. Soc. Sci. Bruxelles A47, 49
- EstablishedGamow (1948). 'The Evolution of the Universe.' Nature 162, 680
- EstablishedAlpher et al. (1948). 'The Origin of Chemical Elements.' Phys. Rev. 73, 803
- EstablishedPenzias & Wilson (1965). 'A Measurement of Excess Antenna Temperature at 4080 Mc/s.' Astrophys. J. 142, 419
- EstablishedPlanck Collaboration (2020). 'Planck 2018 Results. VI. Cosmological Parameters.' Astron. Astrophys. 641, A622,331 citations
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